docs: def calc_anchors(preregistration_map, model, model_hemi,
                   scale=1, sigma=Ellipsis, radius_weight=0, field_sign_weight=0,
                   invert_rh_field_sign=False):
      '''
      calc_anchors is a calculator that creates a set of anchor instructions for a registration.
  
      Required afferent parameters:
        @ invert_rh_field_sign May be set to True (default is False) to indicate that the right
          hemisphere's field signs will be incorrect relative to the model; this generally should be
          used whenever invert_rh_angle is also set to True.
  
      '''
      wgts = preregistration_map.prop('weight')
      rads = preregistration_map.prop('radius')
      if np.isclose(radius_weight, 0): radius_weight = 0
      ancs = retinotopy_anchors(preregistration_map, model,
                                polar_angle='polar_angle',
                                eccentricity='eccentricity',
                                radius='radius',
                                weight=wgts, weight_min=0, # taken care of already
                                radius_weight=radius_weight, field_sign_weight=field_sign_weight,
                                scale=scale,
                                invert_field_sign=(model_hemi == 'rh' and invert_rh_field_sign),
                                **({} if sigma is Ellipsis else {'sigma':sigma}))
      return ancs